The image of a Scout circle-strafing a Heavy quickly enough to smack him into a stupor with a tiny baseball bat is inherently funny. But it only really gets a belly-laugh when the Scout is a scampering stick-boy in knee-high socks, and his victim a meat-headed brickheap of a man. Character is a catalyst for comedy, and until now multiplayer games just haven't had it. They were already funny, but TF2 just brings it out beautifully, every round.
Valve know we like to mock the dead, dance on graves, hump corpses. So as well as making that mockery more crushing, they've also made a game of it: taunting now roots you to the spot, pulls you out into third-person view to watch yourself swagger, leaving you utterly helpless. You've actually got to make a strategic decision about whether you've got a few seconds to play air guitar on your victim's carcass or not. We've seen chain-reactions of death where a Sniper waves to his unfortunate victim, is shot dead mid-mock by another, who then performs the same taunt - with the same fatal result.
But the idea that your character is a character, with his own personality, is only as relevant as you make it. If you leave the taunt and chat commands alone, you'll only really hear yourself if you're a Heavy: the big guy can't resist cackling deliriously if you're getting a lot of kills, and an extraordinary spree will usually be punctuated with a bellowing "SO... MUCH... BLOOD!"
Most maps kick off with the two teams separated by a metal mesh that lifts after a minute, giving Engineers time to build their defences and everyone else a chance to taunt each other. The result is two rows of people jeering, singing, laughing, braying, dancing and whooping at each other in a cacophony of clashing voices. It's a long-needed outlet for our natural tendency to pre-game smack talk, and it makes the atmosphere of the calm before the storm electric.